Hard tube modulator pulse transformer



p 1959 A. D. HASLEY 2,903,583

HARD TUBE MODULATOR PULSE TRANSFORMER Filed Dec. 24, 1956 2 Sheets-Sheet1 FIG.

' j k '1 mane-mow 20 -0UTPUT PULSE SOURCE u) am: TRON 20 I9 0U7'PUT 3INVENTOR 3-4 x x By A. D. HASLfY ATTORNEY Sept. 8, 1959 A. D. HASLEY2,903,583

HARD TUBE MODULATOR PULSE TRANSFORMER Filed Dec. 24, 1956 2 Sheets-Sheet2 INVENTOR y A. 0. HASLEY A TTORNEV United States Patent HARD TUBEMODULATOR PULSE TRANSFORMER Andrew D. Hasley, Basking Ridge, N.J.,assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., acorporation of New York Application December 24, 1956, Serial No.630,415

3 Claims. (Cl. 250-27) This invention relates to pulse generators of thetype employing pulse transformers and more particularly to pulsers ofthe hard tube type which modulate radar transmitters employingmagnetrons.

There are several advantages to be gained by coupling the output of apulser to the load through a pulse transformer. Some of these advantagesare, for example, matching source and load impedances for maximum powertransfer and inverting the polarity of a pulse. A pulse transformer,however, introduces parasitic efiects of leakage inductance anddistributed capacitance into the circuit of the pulser. These parasiticeffects influence the pulse shape and particularly the leading edge ofthe pulse by causing the rise time of the pulse to be greater than itwould be if the pulser were directly connected to the load. A detailedexplanation of these parasitic effects on pulse shape is described inRadiation Laboratory Series, Pulse Generators, volume 5, chapter 14,section 1, pages 563 to 575.

Heretofore, the parasitic effects of the pulse transformer wereprincipally eliminated by employing special windings which were closelycoupled and utilized relatively few turns. A thorough treatment of thepertinent design procedures is also given in Radiation LaboratorySeries, Pulse Generators, volume 5, chapter 13. It is readily apparent,however, that as the load voltage requirements increase considerablecare and ingenuity are required in using the present design techniquesto prevent insulation breakdown from occurring due to the close couplingof the transformer windings. In short, at high load voltages theprotection measures against insulation breakdown of the pulsetransformer windings are inherently opposed to the design requirementsfor minimization of the pulse transformers parasitic elfeets. Thus, theversatility of such a pulser is limited.

It is an object of the present invention to provide a pulse generatoremploying a pulse transformer which produces an output pulse rectangularin shape regardless of load voltage requirements.

It is a more particular object of the invention to eliminate orcompensate for the parasitic effects of the pulse transformer includedin the circuit of a hard tube pulser.

In accordance with a feature of the invention, a seriesresistor-capacitor circuit, each circuit element having a predeterminedvalue, is properly connected in the circuit of the pulser so as to storeand properly discharge energy over the pulse period to compensate forthe parasitic effects of the transformer thereby enabling the pulser toproduce an output pulse rectangular in shape.

In an illustrative embodiment of the invention a pulse transformer isintermittently connected across a source of direct current by ahigh-vacuum tube responsive to a source of unidirectional controlpulses. A series resistorcapacitor circuit of predetermined values isconnected between the primary and secondary windings so as to be chargedto the source potential during the nonconducting state of the switchingtube. The stored charge is uniquely supplied to the secondary of thetransformer when the high-vacuum tube switch is made conducting tocompensate for the energy removed from the source by the parasiticeffects of the transformer. Accordingly, the output pulse of the circuitappearing across the transformer secondary is rectangular in shape.

The invention will be more fully apprehended from the following detaileddescription taken in conjunction with the appended drawings in which:

Fig. 1 is a schematic circuit diagram of one embodiment of theinvention;

Fig. 1A is a schematic circuit diagram of a second embodiment of theinvention;

Fig. 2 is a simplified circuit diagram of the embodiment of Fig. 1; and

Fig. 3 shows an equivalent circuit giving an indication of the loopcurrents in the circuit of Fig. 2.

Referring to Fig. l, a hard tube pulser including a pulse transformer isshown employing the principles of the invention. A source ofunidirectional control pulses 1 is connected to the grid of ahigh-vacuum tube 2. The combination of source 1 and tube 2 operates as aswitch in that the presence of a pulse causes the tube to be conductiveand the absence of a pulse results in the tube being nonconductive.

A pulse transformer 3 having a 1:1 turns ratio is shown within theblocked out portion of Fig. 1. (Pulsers having transformers with turnsratios other than 1:1 are discussed hereinafter.) The transformercomprises windings 3-1, 3-2, and 3-3. The winding 3-1 is the transformerprimary and is connected between terminals 4 and 5. Windings 3-2 and 3-3form a bifilar secondary winding for use with magnetron devices as iswell understood in the art. The ends of windings 3-2 and 3-3 areconnected between terminals 6 and 7 and terminals 8 and 9, respectively.

The source of energy for the pulser is direct voltage source 10. Thepulsing circuit is formed by connecting the negative side of source 10to the cathode of tube 2, and grounding the connection; connecting theanode of tube 2 to terminal 4; and connecting the positive side ofsource 10 to terminal 5.

A source of alternating current 11 furnishes the heater current for amagnetron 12. Source 11 is connected to heater current for a magnetron12 by way of windings 3-2 and 3-3. This is accomplished by connectingsource 11 to terminals 7 and 9 through transformer 13; and, connectingthe heater of magnetron 12 across terminals 6 and 8. Capacitors 14, 15and 16 are placed across the heater circuit to enable pulse currentappearing in windings 3-2 and 3-3 to divide without affecting the heatercurrent. The anode of magnetron 12 is grounded to complete the secondarycircuit of the pulser.

As shown in Fig. l, a capacitor 17 and a resistor 18 are connected inseries between the terminal 4 and the terminal 6. During each periodbetween pulses there is stored in the capacitor energy which isdelivered to the transformer on the initiation of the pulse period. Theresistor causes this energy to be delivered in proper phase tocompensate for the effects of the transformer leakage reactance. Thevalues of the capacitor and resistor are most important to the properperformance of the pulser, as will be evident hereinafter. Accordingly,it appears logical at this point to explain the choice of values forthese passive circuit elements.

We begin the explanation by indicating the simplified circuit shown inFig. 2. An exact analysis of the circuit shown in Fig. 1 leads toimpractically complicated results. The circuit of Fig. 2, however,represents a satisfactory approximation of the inventions circuit asevidenced by empirical results obtained therefrom.

Referring to Fig. 2, transformer 3 is indicated and is assumed to beideal except for leakage inductance L.

Further, no energy is stored in the leakage inductance L prior toswitching the battery into the primary circuit. The resistance ofmagnetron 11 or other load device is shown as R The space pathresistance of tube 2 is shown as bR where b is a constant to be laterdetermined. The tube resistance is assumed constant during theconducting period. Voltage source 10 is shown on Fig. 2 as E Capacitor17, whose value is to be determined, is shown as C. The capacitor isassumed to be fully charged before switching. Resistor 18 is shown aszzR where a is a constant representing the ratio of R to the resistanceof resistor 18.

The circuit of Fig. 2 appears as Fig. 3 after tube 2 is switched intothe conducting state. The loop equations of the circuit written indifferential form are:

malaria-Efrem RLi.+ edi= -Ebb subject to conditions: t 3(0) =0 (3Substituting condition (0) into Equations 1 and 2 and rewriting producesthe following:

t 1 +a+b vl+% fl ventila ed tam-Ebb Letting the Laplace transforms (V =V(s) and i I (s), then:

For the output voltage v to be constant the coefficients of like powersof s in the quadratics appearing in the numerator and denominator ofEquation 8 must be equal. It may be readily shown that this conditioncannot be met exactly. If it is assumed that b=a and that b 1 and (a+b)1, it may be met approximately. Thus,

and

L aRL T (1+2a)LC (10) where Since a=b, then In designing the pulser thefirst step is the selection of high-vacuum tube 2. It will be rememberedthat the tube resistance is bR where b 1. Accordingly, a tube isselected having a resistance much less than the load resistance. As anexample, a high-vacuum tube having a space path resistance of 50 ohmsshould be employed with a magnetron having an effective load resistanceof 1200 ohms. The value of resistor 18 should also be 50 ohms since, asindicated above, aR =bR The value of capacitor 17 is obtained byestimating or measuring the leakage inductance of transformer 3 andsubstituting this value and the tube resistance value into Equation 12.These chosen values of resistor 18 and capacitor 17 enable the outputpulses of the circuit shown in Fig, l to be rectangular in shape inaccordance with the mathematical proof outlined above.

Returning to Fig. l, the operation of the invention will be described indetail. In one condition of the pulser, tube 2 is nonconducting due tothe absence of a pulse from source 1. During this period capacitor 17 isin series with source 10 and charges to the potential of the source. Theheater of magnetron 11 is energized from source 11, but no pulse energyis received from transformer 3 snice winding 3-1 is disconnected fromsource It).

In the next condition of the pulser, tube 2 is made conductive by theappearance of a pulse from source 1. As a consequence, source 10 isconnected across winding 3-1 and current flow commences in the primarycircuit of the pulser. Since magnetron 11 is a biased diode type ofdevice, there is no load current flowing until the voltage across theprimary and secondary of transformer 3 exceeds the equivalent biasvoltage of the magnetron. Ordinarily, the buildup of the source voltageacross magnetron 12 is less than the buildup of the voltage acrosswinding 3-1 due to the fact that a portion of the source currentreflected into windings 3-2 and 3-3 is diverted by the leakageinductance and distributed capacitance of transformer 3. However, thepresent invention enables the voltage across the magnetron to rise asfast as the voltage across winding 3-1 as explained hereinbelow.

The conduction on the part of tube 2 connects capacitor 1'7 and resistor18, to ground (through space path of tube) and therefore in parallelwith windings 3-2 and 3-3. Hence, the voltage across capacitor 17, whichis the same as that of source 10, is applied across windings 3-2 and3-3, and, in contrast to conventional pulser, the equivalent bias ofmagnetron 12 is overcome without delay when tube 2 is made conducting.Load current commences to flow in the secondary of transformer andcapacitor 17 supplies the current difference between primary andsecondary windings temporarily lost to the parasitic effects of thetransformer. It is obvious that the values of capacitor 17 and resistor18 are most important to the proper performance of the pulser sinceenergy delivered by capacitor 17 which is more or less than therequirements of the leakage inductance, etc. will alter the desiredpulse shape. However, as the result of the design considerationsoutlined above, the relative values of the capacitor and resistor arechosen to control the flow of energy from capacitor 17 in such phase asto compensate for the energy diverted to the trans formers parasiticelements.

After the parasitic effects of the transformer have been compensated,capacitor 17 is recharged by source 10 so that when tube 2 is madenonconducting the capacitor is essentially fully charged and ready forthe next switching operation. During this charging period capacitor 17acts to maintain the top of the pulse essentially flat by absorbingenergy in excess of the pulse requirements or supply ing energy when thepulse energy is insufiicient.

Fig. 1A shows another embodiment of the invention where the turns ratioof transformer 3 is other than lzl. However, as shown in Fig. 1, theresistor-capacitor circuit is connected to the secondary of transformer3 so that the lower portion of the secondary winding has the same numberof turns as the primary of transformer 3. In the event that theresistor-capacitor circuit was connected to some other point on thesecondary winding, it would produce more complicated formulae fordetermining the values of the elements of the resistor-capacitorcircuit. Hence, the 1:1 tap on the secondary is arbitrarily selected forreasons of convenience.

In Fig. 1A the arrangement of the secondary windings producesautotransformer action with windings 3-2 and 3-3 forming the primary andwindings 3-4 and 3-5 forming the secondaries of the autotransformer,respectively. The voltage buildup across the autotransformers will bedegraded from the ideal condition due to the parasitic effects of theautotransformer. However, it is evident that there is better couplingbetween the sections of the autotransformer windings than between theprimary and the whole secondary winding of transformer 3. Thus thecircuit of Fig. 1A operates the same as Fig. 1 except that it producesvoltage step-up of the pulse with some parasitic elfects. Coupling theautotransformer winding as closely as possible substantially eliminatesthese parasitic effects.

It will be evident from the foregoing that the series resistor-capacitorcircuit is relatively simple to select and install on a transformerincluded in the circuit of a pulser. The improvement in pulse shapeobtained therefrom also enables the pulser to be operated at fasterpulse repetition rate than for pulser without the modification since thevoltage buildup across the load device is faster than that for prior artdevices.

The series resistor-capacitor circuit is also applicable to low voltagetransformers as well as in the case of the described high voltageapplications. Thus, the invention in its broad application offersconvenient means for effecting a number of desirable improvements in thecharacteristics of all pulsing circuits employing transformers.

It is to be understood that the above-described arrangements are merelyillustrative of the principles of the invention, and applicant does notintend to limit the invention of the particular embodiments shownherein. Numerous other embodiments may be devised by those skilled inthe art without departing from the spirit and scope of the invention.

What is claimed is:

l. A pulse generator comprising a pulse transformer having primary andsecondary windings, said primary having first and second terminals, acircuit including a capacitor and a resistor connected between the firstterminal and the secondary, an output circuit connected across saidsecondary, a source of direct current, and switching means operableintermittently for simultaneously connecting said capacitor and resistorcircuit across the secondary and said source between the first andsecond terminals, the resistance of the switching means in its closedcondition being bR where R is the resistance of a load connected acrossthe output circuit and b is a constant subsequently less than one, thecapacitor having a value of the transformers leakage inductance dividedby the square of bR and the resistor having a. value of bR 2. A pulsegenerator comprising a pulse transformer having primary and secondarywindings, said primary having first and second terminals, said secondarybeing tapped at a point corresponding to the number of turns on theprimary, an output circuit having one side at ground potential beingconnected across the secondary, a source of direct current connectedbetween ground and the second terminal, a circuit including a capacitorand resistor connected between the first terminal and the secondary tap,and switching means operable intermittently connected between ground andthe first terminal.

3. A pulse generator comprising a pulse transformer having primary andsecondary windings with a leakage inductance L, a load circuit ofresistance R connected to said secondary winding, a source of directcur-rent having one terminal connected to a first terminal of saidprimary winding and a second terminal connected to a point of voltagecommon to one terminal of said secondary winding, a vacuum tubeswitching device having a space path resistance in the conductingcondition of bR where b is a constant substantially less than unity, andconnected between said second terminal of said source of direct currentand a second terminal of said primary winding, means for intermittentlyrendering said switching device conductive to produce pulses in saidsecondary winding and in said load circuit during the conductiveperiods, and a capacitor of capacitance C divided by the square of bRand a resistor of resistance bR connected in series between said secondterminals of said primary and secondary windings.

References Cited in the file of this patent UNITED STATES PATENTS

